What is the test for determining when a statute operates retroactively?

As the United States Supreme Court has recognized, "deciding when a statute operates 'retroactively' is not always a simple or mechanical task" (Landgraf v. USI Film Products(1994) 511 U.S. 244, 268, 114 S.Ct. 1483, 128 L.Ed.2d 229) and "comes at the end of a process of judgment concerning the nature and extent of the change in the law and the degree of connection between the operation of the new rule and a relevant past event" (id. at p. 270, 114 S.Ct. 1483). In exercising this judgment, "familiar considerations of fair notice, reasonable reliance, and settled expectations offer sound guidance." (Ibid.)